A typical wireless communication system includes a number of base stations each radiating to provide coverage in which to serve wireless communication devices (WCDs) such as cell phones, tablet computers, tracking devices, embedded wireless modules, and other wirelessly equipped devices. In turn, each base station may sit as a node on a core access network that includes entities such as a network controller and a gateway system that provides connectivity with an external transport network such as the Internet. With this arrangement, a WCD within coverage of the system may engage in air interface communication with a base station and may thereby communicate via the base station with various remote network entities or with other WCDs served by the base station.
Such a system may operate in accordance with a particular air interface protocol, examples of which include, without limitation, Long Term Evolution (using Orthogonal Frequency Division Multiple Access (OFDMA) and Single Carrier Frequency Division Multiple Access (SC-FDMA)), Code Division Multiple Access (CDMA) (e.g., 1×RTT and 1×EV-DO), Global System for Mobile Communications (GSM), IEEE 802.11 (WiFi), BLUETOOTH, and others. Generally, the air interface protocol may define a downlink (or forward link) for carrying communications from a base station to WCDs served by the base station, and an uplink (or reverse link) for carrying communications from the WCDs to the base station.
In practice, a base station and its served WCDs may utilize various air interface resources to facilitate communication on the downlink and the uplink. By way of example, an LTE air interface on both the downlink and the uplink may span a particular carrier frequency bandwidth (such as 1.4 MHz, 3 MHz, 5 MHz, 10 MHz, 15 MHz, or 20 MHz) that is divided over time into a continuum of 10-millisecond frames each defining ten 1-millisecond subframes or time transmission intervals (TTIs), and each TTI is divided over time into 14 symbol time segments of 66.7 microseconds in which data can be transmitted. Further, the carrier bandwidth is divided into a sequence of 15 kHz subcarriers. As a result, in each TTI, the LTE air interface defines an array of “resource elements” in which data can be communicated.
When the base station has data to transmit to one or more served WCDs, the base station may transmit that data to the WCD(s) at a given moment in time—namely, in a given symbol time segment within a TTI. For instance, the base station could engage in one transmission in a given symbol time segment. Alternatively, the base station can engage in multiple concurrent transmissions in a given symbol time segment. For instance, the base station may engage in multiple concurrent transmissions in the symbol time segment to a single WCD, or may engage in multiple concurrent transmissions in the symbol time segment to multiple WCDs.
The base station may be configured to operate at a particular total transmit power level. This total transmit power level may define for the base station a cumulative transmit power level that the base station can use across the carrier bandwidth in a given symbol time segment. For instance, if the base station engages in one transmission in a given symbol time segment, the base station will engage in that transmission at the total transmit power level. Whereas, if the base station engages in multiple concurrent transmissions in a given symbol time segment, the total transmit power level may be divided among those concurrent transmissions. Further, the total transmit power level could be fixed over time (e.g., the base station may be set to transmit at the same total transmit power level in each symbol time segment) or could vary per symbol time segment or per TTI. And still further, the total transmit power level could be divided equally across the carrier bandwidth among the subcarriers, or could be distributed in some other manner across the bandwidth.
In practice, the base station may also be configured such that the total transmit power level per symbol time segment (or on average per unit time) cannot exceed a specified maximum total transmit power level. This maximum total transmit power level could be specified by a manufacturer, by government regulations, or in some other manner.
In addition, a typical base station may include an antenna structure having multiple transmit antennas, and the base station may be configured to use a particular quantity of those transmit antennas for transmitting to served WCDs. As such, when the base station transmits at its total transmit power level in a given symbol time segment, the total transmit power level will be distributed among that quantity of transmit antennas.
Data communication between a base station and a served WCD over the air interface may operate in accordance with a coding scheme for encoding the data into an encoded bit sequence at the transmitting end, and for correspondingly decoding the encoded bit sequence to uncover the underlying data at the receiving end. Such data communication may also operate in accordance with a modulation scheme that establishes how the bits of the encoded sequence will be modulated onto a carrier signal at the transmitting end, and thus how the bits will be demodulated from the carrier signal at the receiving end. In particular, the modulation scheme may provide for mapping groups of bits from the encoded sequence into symbols that represent phase, amplitude, and/or other air interface characteristics, and then modulating the symbols onto the carrier signal at the transmitting end. This mapping may be defined by a constellation pattern made up of various constellation points, with each point representing a respective symbol.
In practice, modulation schemes may range from low-order to high-order, in terms of how many bits can be mapped per symbol. For instance, higher order modulation schemes map more bits per symbol than lower order modulation schemes, and thus their constellation patterns are more dense and granular than lower order modulation schemes.
When a base station serves a WCD, the base station may select a modulation scheme to use for their air interface communication, with the selection being based on various factors, such as the quality of the WCD's air interface connection with the base station. For instance, the WCD may regularly evaluate its air interface channel conditions and provide the base station with a channel quality report, and the base station may then select a highest-order modulation scheme that is compatible with the WCD's reported channel conditions. If the WCD has poor channel conditions, then the base station may select a relatively low-order modulation scheme, in which case the rate of data communication per resource element would be relatively low. Whereas, if the WCD has good channel conditions, then the base station may select a relatively high-order modulation scheme, in which case the rate of data communication per resource element would be relatively high.